The present invention relates to a low noise converter (Low Noise Block Downconverter: hereinafter referred to as utilized, for example "LNB")for receiving at least two frequency bands from a satellite television broadcast down link, and more particularly, to a construction of an LNB for switching a receiving band in accordance with a predetermined change in the level of a power-supply voltage.
Conventionally, as is shown in FIG. 2, and as further shown and described in applicant's related copending application Ser. No. 07/200,558, entitled "Outdoor Unit Low Noise Converter For Satellite Broadcast Reception Use", filed on May 31, 1988 a power-supply voltage fed through an IF output terminal, is sensed by a coil L. The sensed power-supply voltage is divided by resistors R1 and R2, and is compared with a reference voltage by a comparator 2. The reference voltage is coupled through a voltage stabilizer from the power-supply voltage.
When the output of the comparator 2 is higher than the reference voltage and higher than the threshold voltage of a transistor TR, the transistor TR is switched on i.e. rendered. By this action a relay 3 operating as a switch relay switches the receiving frequency band to another band of receiving frequencies. R3 is a resistor to effect a stable hysteresis characteristic. D is a diode to protect the transistor TR. C is a condenser to block a direct current. The, output of a three-terminal regulator 4 is supplied to each circuit, not shown, inside the LNB.
It is to be noted, however, that with the above conventional circuit, the relay 3 is operated when the switching power-supply voltage level is high. This causes an increase of power consumption and further causes a temperature rise inside the LNB.